1. Field of the Invention
The present invention relates to an optical fiber amplifier for optically amplifying a light signal and an optical fiber transmission apparatus using the optical fiber amplifier.
2. Description of the Related Art
In a conventional analog light transmission apparatus, a laser diode oscillating at a wavelength in the 1.3 .mu.m band has been used to generate signal light. With a recent development of an optical fiber amplifier doped with erbium (Er) ions capable of optically amplifying light having a wavelength in the 1.55 .mu.m band, an analog light transmission system using a signal falling within this wavelength bandwidth has been developed.
FIG. 14 shows a configuration of such an analog optical fiber transmission apparatus for transmitting light of the 1.55 .mu.m wavelength band. Referring to FIG. 14, signal light (wavelength: 1.55 .mu.m band) emitted from a DFB (distributed feedback) laser 30 is optically amplified by an Er-doped optical fiber amplifier 31. The amplified signal light propagates through a single-mode optical fiber 32 having a length of 10 km and then detected by a light receiver 33. The DFB laser 30 allows the output thereof to be modulated in accordance with an AM-FDM (frequency-division-multiplex) or FM-FDM analog signal. The Er-doped optical fiber amplifier 31 is pumped by laser light (wavelength: 1.48 .mu.m band) emitted from a pumping laser diode (not shown). A photodiode made of InGaAs or an avalanche photodiode is used for the light receiver 33.
An Er-doped optical fiber amplifier provides a high gain for light of the 1.55 .mu.m wavelength band. Thus, by using the Er-doped optical fiber amplifier as a booster amplifier, a totally optical distribution system where all lines from head ends to end users are implemented by optical fibers, can be constructed. This makes it possible to realize an image distribution service covering 100 or more channels and a high-definition TV image distribution service.
The modulation at the laser diode for emitting signal light as mentioned above not only modulates the power of the signal light emitted by the laser diode but also changes the frequency thereof. This phenomenon is called "chirping".
The Er-doped optical fiber amplifier provides a high gain for the light of the 1.55 .mu.m wavelength band, as described above. However, since the gain has a wavelength-dependency (gain tilt), the following problems arise due to the chirping.
When signal light whose power and frequency have both been modulated passes through the optical fiber amplifier, it is subjected to a secondary intermodulation (IM2) distortion caused by the frequency modulation and the gain tilt. As a result, the signal light distorts significantly. In general, the degree of distortion of the signal light at the optical fiber amplifier varies considerably depending on the power of the signal light input into the optical fiber amplifier.
In the case where a plurality of signal light beams with different wavelengths are amplified by a single optical fiber amplifier, the distortion characteristic varies considerably depending on the number of signal light beams, the wavelength, and the input power. This is because the gain and the gain tilt vary depending on these factors.